Double modulation radio receiver



May 14, 1946. G. E. PRAY DOUBLE MODULATION RADIO RECEIVER 2 Sheets-Sheet 1 Filed March 27, 1942 AF AMPLIFIER 0U TPU T NAP/POW BAND PRl. I MOD. FREQ- +DE7'EC7'0R SEL ECTOR l 6 AMP! 1 v DETECTOR BROAD BAND HIGH. FREQ. l-F AMPLIFIER SUPER FAQEQ. PPE- F/?EQ- SELECTOR M/XE'I? SUPER FREQ. HE F EPODYNE o SUPER INPUT INVENTOR GTEmer's nPr'a NARROW BAND /F 4 AMPLIFIER 7 CW OSC.

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Filed March 27, 1942 2 Sheets-Sheet 2 SE: n

INVENTOR I (ZEmer #Omv v0.00 OuuE Fins \mw 1 m rnlll rlLlil lllll tokuwkhq E 93th kuiimtw 5 93.5w Qmiilt Patented May 14, 1946 UNITED STATES PATENT OFFICE- DOUBLE MODULATION RADIO RECEIVER George Emerson Pray, Clearfield, Pa. Application March 27, 1942, Serial No. 436,470

8 Claims. (01. 250- 20) (Granted under the m of'March 3, issaas' amended April so, 1923; 370 o. G. 757) This invention relates to a novel means for the efiicient reception of radio signals of the double modulationtype.

Among the several objects of this invention are:

To provide a system of reception that will permit the use of several primary and secondary modulation frequencies simultaneously on the same carrier;

To provide a receiving system of the type specified of such nature that carrier frequency drift or instability is relatively unimp ortant;

To rovide a method of continuous wave telegraph communication at very high frequencies;

To provide means for efficiently receiving double modulation signals.

Other objects and advantages will become apparent from the detailed description of the present invention.

In the drawings:

Fig. l is a block diagram of one system embodying the present invention;

' Fig. 2 is a block diagram of sections that may be substituted in the diagram of Fig. 1 for certain purposes that will be hereinafter set forth;

and

Fig. 3 is a schematic circuit diagram of present invention. 1

At high radio frequencies of the order of 100 megacycles per second or more, considerable difficulty has been encountered in maintaining constant frequency oscillations in transmitters and receivers, with the result that a desired signal frequency may drift or frequenc modulate through receiver resonance at either a slow or a rapid rate. This results in distortion and fading, and the complete impracticability of CW reception of the carrier. i

Double modulation overcomes these difficulties, as well as presenting other advantages of value. Double modulation is the process of producing a high radio frequency carrier signal, modulated by a low frequenc or primary modulation, which in turn is modulated by audio frequency or'secondary modulation. The resulting signal includes components of all the frequencies applied. If an ordinary single modulation receiver is. tuned to one of the sidebands of such a signal, it is possible to receive and interpret the audio signal, though with poor efficiency due to the small amount of audio energy present in that sideband compared to the total energy transmitted. If two or more primary and secondary modulation frequencies are present, such a receiver would normally be unable to discriminate between them,

merely reproducing all the applied audio frequencies simultaneously,

In a double modulation receiver the preselector is resonated at the carrier frequency, and is. designed with a sufficiently broad pass band to take in the sidebands that include the primary and secondary modulation frequencies, plus a reasonable allowance for carrier drift or instability. The signal is then rectified to reproduce the primary modulation frequency, and passed through a narrow band amplifier. This frequency is then rectified to reproduce the audio or secondary modulation which may be further amplified as desired.

Fundamentally, the present invention consists of a superfrequency preselector I, a superfrequency mixer 2, into which feeds the output of a superfrequency heterodyne oscillator 3. The output of superfrequency mixer 2, which is a first intermediate frequency, is fed into a broadly tuned amplifier 4 whereof the output is transferred to a detector 5 which feeds the primary modulation carrying the secondary modulation into a narrow band primary modulation frequency selector and amplifier 6. The detector 8 connected to selector and amplifier 6 detects the secondary modulation and transfers it to audio frequency amplifier 9. Continuous wave oscillator 1 may be connected to selector and amplifier l3 when the signal to be received is a continuous wave keyed with tele v graphic signals instead of carrying a secondary audio frequency modulation.

The sections H), II, I2 and 13, shown in Fig. 2, may be substituted for sections. 6 and l in Fig. 1, if it is desired to utilize a second heterodyne 0scillator feeding into the primary modulation fre-. quency selector and mixer I 0, the section l2 being merely a closely tuned amplifier for the second intermediate frequency thus generated.

For purposes of discussion and illustration, but without in an way imposing limitations, it will be assumed that the carrier frequency is 200.

fled signal from 15 is transferred to SUPGIfIB': quency mixer'tube H! where it beats with the out-f putcf superfrequency oscillator 3.to produce a first intermediate frequency of 13 mega'cycles. This first intermediate frequency carries the double modulation through the broadband amplifier 4, which may comprise a transformer I9 and amplifier tube 20, a transformer 2| and a second amplifier 22 all connected in series. The amplified first intermediate frequency is transferred by transformer 23 to a detector tube 24 which detects the primary modulation carrying the secondary modulation as side bands.

The rectified signal from tube 24 is fed into the narrow band primary modulation frequency selector and amplifier 6, which is resonated at the primary modulation frequency and has only sufli cient band width to pass the primary modulation with the secondary modulation frequency carried thereby. The amplified signal is transferred by conductor 25 to detector 8 which rectifies the voice frequency signal and this in turn is amplified in the tubes 26 and 2'! of the audio frequency amplifier 9 and thence to the output circuit 28. When so used the continuous wave oscillator is held inoperative.

If it is desired to communicate by signals keyed upon the continuous wave primary modulation, continuous wave oscillator I is connected into the circuit by closing switch 29 so that an audio frequency beat note is present in the signal fed into detector 8.

The transmission of several primary modulation frequencies carrying their respective secondary modulation. frequencies on a single carrier wave is frequently desirable, which primary modulation frequencies need not be separated by more than about 10 kilocycles. The selection of any particular primary modulation with its respective secondary modulation is effected by changing the setting of the frequency selector units 30, 3| and 32.

As hereinbefore stated, the sections shown in Fig. 2 may be substituted for sections 6 and 'l in Fig. 1. It appears unnecessary to illustrate the circuits of the primary modulation frequency selector and mixer I and the second heterodyne oscillator II, since such circuits are well known in the art. The primary modulation frequency from detector is fed into the selector and mixer I0 where it beats with the output of second heterodyne oscillator I I to produce a second intermediate frequency of relatively low frequency which is amplified in the narrow band second intermediate frequency amplifier I2. If the signal is keyed upon the primary modulation, then switch 33 is closed so that continuous wave oscillator I3 produces an audio frequency beat note in the output of amplifier I2.

It will be seen from the foregoing that if the preselector I and first intermediate frequency amplifier 4 have sufliciently broad pass band characteristics, a change or drift of the carrier frequency within this pass band will have no effect upon the audio output of the receiver and no effect upon the frequency of the primary modulation or upon the continuous wave beat note of such modulation. The stability of the primary modulation frequency permits using a selector and amplifier 6 that may have a narrow pass band which will greatly reduce the noise component present in the signal. Since the primary modulation frequency is highly stable, several such primary modulation frequencies each carrying a secondary modulation frequency may be transmitted on the same superfrequency carrier and so provide several communication chan nels where only one would otherwise be available.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes Without the payment of any royalities thereon or therefor.

I claim:

1. A receiver for a superfrequency carrier wave modulated with a primary and a secondary modulation, comprising a superfrequency preselector section, a superfrequency mixer fed from said section, a superfrequency heterodyne oscillator connected to said mixer to produce a high frequency intermediate frequency in the output of said mixer, a first amplifier section broadly tuned to said intermediate frequency connected to transfer said output, a first detector fed from said amplifiersection to detect said primary modulation, a second amplifier section highly selective for said primary modulation connected to said detector, a second detector for said secondary modulation-connected to the output of said second amplifier, and an amplifier for said secondary modulation connected to the output of said second detector.

2. A receiver for a superfrequency carrier Wave modulated with a primary and a secondary modulation, comprising a superfrequency preselector section, a superfrequency mixer fed from said section, a superfrequency heterodyne oscillator connected to said mixer to produce a high frequency intermediate frequency in the output of said mixer, a first amplifier section broadly tuned to said intermediate frequency connected to transfer said output, a first detector fed from said amplifier section to detect said primary modulation, a second amplifier section highly selective for said primary modulation connected to said detector, and a second detector for said secondary modulation connected to the output of said second amplifier.

3. A receiver for a superfrequency wave carrying a modulation that is keyed to constitute si nals, comprising a superfrequency preselector section, a superfrequency mixer fed from said section, a superfrequenc heterodyne oscillator connected to said mixer to produce a high frequency intermediate frequency in the output of said mixer, a first amplifier section broadly tuned to said intermediate frequency connected to transfer said output, a first detector fed from said amplifier section to detect said modulation, a second amplifier section highly selective for said modulation connected to said detector, a continuous Wave oscillator connected to said second amplifier section to produce an audio beat note in the output of said second amplifier section, and a second detector to detect said audio beat note,

4. A receiver for a superfrequency carrier wave modulated with a primary and a secondar modulation, comprising a superfrequency preselector section, a superfrequency mixer fed from said section, a superfrequency heterodyne oscillator connected to said mixer to produce a high frequency intermediate frequency in the output of said mixer, a first amplifier section broadly tuned to said intermediate frequency connected to transfer said output, a first detector fed from said'amplifier section to detect said primary modulation, a primary modulation selector and mixer fed from said detector, a second heterodyne oscillator connected to said primary modulation mixer to produce a lower intermediate frequency, a nar row band amplifier connected to the output of said primary modulation mixer, a second detector connected to detect said secondary modulation,

and an amplifier section connected to the output of said detector.

5. A receiver for a superfrequency carrier Wave modulated with a primary and a secondary modulation, comprising a superfrequency preselector section, a superfrequency mixer fed from said section, a superfrequency heterodyne oscillator connected to said mixer to produce a high frequenc intermediate frequency in the output of said mixer, a first amplifier section broadly tuned to said intermediate frequency connected to transfer said output, a first detector fed from said amplifier section to detect said primar modulation, a primary modulation selector and mixer fed from said detector, a second heterodyne oscillator connected to said primary modulation mixer to produce a lower intermediate frequency, a narrow band amplifier connected to the output of said primary modulation mixer, and a second detector connected to detect said secondary modulation.

6. A method of receiving signals transmitted as a secondary modulation impressed upon a primary modulation on a superfrequency carrier wave train, comprising the steps of selectively receiving said carrier, beating the received wave with a local high frequency wave to produce a high frequency intermediate frequency, said intermediate frequency lying between the superfrequency carrier and the primary modulation, amplifying said intermediate frequency, detecting said primary modulation, selecting and amplifying said primary modulation, detecting said secondary modulation, and amplifying said detected secondary modulation.

7. A method of receiving signals transmitted as a secondary modulation impressed upon a primary modulation on a superfrequency carrier wave train, comprising the steps of selectively receiivng said carrier, beating the received wave with a local high frequency wave to produce a high frequency intermediate frequenc said intermediate frequency lying between the superfrequency carrier and the primary modulation, amplifying said intermediate frequency, detecting said primary modulation, selecting and amplifying said primary modulation, and detecting said secondary modulation.

8. A method of receiving signals keyed on a modulation impressed upon a superfrequency wave train, comprising the steps of selectively receiving said wave train, beating the received wave with a local high frequency wave to produce a high frequency intermediate frequency, said intermediate frequency lying between the superfrequency carrier and the primary modulation, amplifying said intermediate frequency, detecting said modulation, beating saidv detected modulation to produce an audio signal, and detecting said audio signal.

GEORGE E. PRAY. 

